1. Field of the Invention
The present invention relates to an apparatus and a process for recovering abrasive. More particularly, the present invention relates to an apparatus and a process for recovering and reusing particles of abrasive from a waste fluid containing the abrasive which is discharged from the chemical mechanical polishing process (the CMP process) in semiconductor manufacturing factories.
2. Description of Related Art
It is required that the surface of coating films such as insulating films and thin films of metals which are formed on semiconductor wafers be a highly flat surface. As the means of forming the flat surface, the CMP process is conducted. In this process, polishing is conducted in the condition such that a slurry of abrasive is present between a polishing tool such as a polishing pad and a semiconductor wafer. As the abrasive used in the CMP process, fine particles of silica exhibiting excellent dispersion and having a uniform distribution of the particle diameter, fine particles of ceria exhibiting a great polishing rate and fine particles of alumina having a high hardness and stable are used. These abrasives are supplied by manufacturers as slurries in which particles having a prescribed diameter are dispersed in water in a prescribed concentration and are used after being diluted into a prescribed concentration for the actual use. In general, an agent for adjusting pH such as potassium hydroxide, ammonia, organic acids and amines, surfactants as the dispersants and oxidizing agents such as hydrogen peroxide, potassium iodate and iron(III) nitrate are added to the slurry in advance or at the time when the slurry is used for polishing.
Since the polishing slurry is used in a great amount and expensive and the amount of industrial waste materials must be decreased, reuse of the polishing slurry is desired. However, the waste fluid of the CMP process has a low concentration of the abrasive due to dilution and contains dusts formed from polishing pads and coating materials, fine particles formed by fracture of the abrasive and solid impurities having great diameters formed by aggregation of abrasive particles. When this waste fluid of the CMP process is reused as the abrasive for the polishing without treatments, problems arise in that the polishing rate decreases due to the decrease in the concentration of the abrasive and that the surface of wafers is damaged. Moreover, since additives are used in the abrasive, the waste fluid of the CMP process contains the additives left remaining in the fluid. Impurities of metal ions formed by the polishing are also contained. When the recovered slurry is reused, the impurities causes difficulty in the adjustment of the concentration and the concentration cannot be controlled readily. Due to the above reasons, the waste fluid of the CMP process cannot be recycled for reuse without treatments. When the waste fluid of the CMP process is reused, it is necessary that the waste fluid of the CMP process be treated for removing impurities such as coarse particles and salts from the fluid and be concentrated so that the slurry of the abrasive having the prescribed composition is prepared again.
Heretofore, various developments on the technology for the treatment of a waste fluid of the CMP process have been attempted. For example, in a process disclosed in Japanese Patent Application Laid-Open No. Heisei 10(1998)-118899, a waste fluid of the CMP process is treated by a microfiltration membrane to remove coarse particles and then by ultrafiltration membrane, agents are added to the treated fluid to adjust the concentration and the obtained fluid is reused as the slurry of the abrasive. The coarse particles can be removed and the damages on the surface of wafers can be suppressed in accordance with the above process. However, since the concentration of the abrasive and pH are adjusted by adding fresh agents while the additives and salts in the waste fluid are left remaining, these impurities cause a problem in the adjustment of the concentration and the adjustment of the concentration cannot be achieved satisfactorily. Moreover, adverse effects such as contamination of the product are exhibited. In particular, when an organic dispersant is used to control the distribution of the particle diameter so that the distribution of the particle diameter of the abrasive in the slurry is kept within a prescribed range, the concentration of the above impurities affects the distribution of the particle diameter to a great extent and it is important that the residual agents and salts are removed. When a slurry of a silica-based abrasive is used, an alkaline agent is used to adjust the distribution of the particle diameter. Since pH of the slurry decreases due to dilution by the washing with ultrapure water after the polishing in accordance with the CMP process, the alkaline agent is added to optimize the distribution of the particle diameter. However, even when potassium hydroxide is added after coarse particles have been removed in accordance with a conventional process, pH cannot be adjusted sufficiently due to the difficulty caused by the residual salts and, therefore, the distribution of the particle diameter cannot be adjusted sufficiently. In particular, when a slurry of a silica-based abrasive is used, the behavior exhibited by the addition of a solution of potassium hydroxide in the presence of a colloidal substance is different from that in the absence of colloidal substances. For example, potassium hydroxide is consumed for dissolving and converting colloidal silica into soluble silica and for forming hydroxyl group on the surface of colloidal silica. Therefore, the relation between the concentration of the added potassium hydroxide and pH varies depending on the condition and cannot be used as the reference to the operation. Therefore, the removal of the residual colloidal substances is important when a slurry of a silica-based abrasive is used.
In a process disclosed in Japanese Patent Application Laid-Open No. 2000-288935, a waste fluid of the CMP process is treated by a microfiltration membrane to remove coarse particles, the obtained filtrate is concentrated by centrifugation, the concentrated fluid is washed with water and, where necessary, the size of particles, the concentration and pH are adjusted. However, this process has drawbacks in that the concentration of the abrasive is not sufficiently increased and that, when the fluid is treated by strong centrifugation to increase the concentration, the suitable size as the abrasive cannot be maintained due to an increase in the size of the abrasive or solidification of the abrasive and the load to the means of membrane separation in the following step markedly increases.
The present invention has an object of providing an apparatus and a process for efficiently recovering and reusing particles of abrasive from a waste fluid containing the abrasive which is discharged from the CMP process in semiconductor manufacturing factories.
As the result of intensive studies by the present inventors to achieve the above object, it was found that impurities such as salts and organic substances in a waste fluid of the CMP process could be effectively separated and a slurry comprising particles of the abrasive having a high purity and a suitable diameter could be recovered when a combination comprising a means of pre-filtration by which the entire amount of the waste fluid was filtered and coarse particles were removed, a means of membrane separation in which the filtered waste fluid obtained from the means of pre-filtration was introduced and concentrated, a means of washing by which the concentrated slurry obtained from the means of membrane separation was washed and a means of post-filtration by which the slurry concentrated by the means of membrane separation and washed by the means of washing was filtered and the size of the particles of the abrasive was adjusted as the last step was used, the concentrated slurry obtained by the means of membrane separation was diluted and dispersed again by adding water and the obtained slurry was washed with water and concentrated again by the means of membrane separation. The present invention has been completed based on this knowledge.
The present invention provides:
(1) An apparatus for recovering abrasive from a waste fluid of a CMP process, the apparatus comprising a means of pre-filtration into which the waste fluid of a CMP process is introduced, a means of membrane separation into which a filtered waste fluid obtained from the means of pre-filtration is introduced, a means of washing by which a concentrated slurry obtained from the means of membrane separation is washed with water and a means of post-filtration by which the concentrated slurry obtained from the means of washing is filtered;
(2) An apparatus described in (1), which comprises a means of adjustment of at least one of pH and a concentration disposed between the means of membrane separation and the means of post-filtration or after the means of post-filtration;
(3) An apparatus described in (1), wherein the means of membrane separation is a membrane separation apparatus comprising a microfiltration membrane having a pore size of 0.05 to 0.2 xcexcm;
(4) An apparatus described in (2), wherein the means of membrane separation is a membrane separation apparatus comprising a microfiltration membrane having a pore size of 0.05 to 0.2 xcexcm;
(5) An apparatus described in (1), wherein the means of pre-filtration and the means of post-filtration are microfilters of a dead end filtration type equipped with a microfiltration membrane having a pore size of 10 to 100 xcexcm;
(6) An apparatus described in (2), wherein the means of pre-filtration and the means of post-filtration are microfilters of a dead end filtration type equipped with a microfiltration membrane having a pore size of 10 to 100 xcexcm;
(7) An apparatus described in (3), wherein the means of pre-filtration and the means of post-filtration are microfilters of a dead end filtration type equipped with a microfiltration membrane having a pore size of 10 to 100 xcexcm;
(8) An apparatus described in (4), wherein the means of pre-filtration and the means of post-filtration are microfilters of a dead end filtration type equipped with a microfiltration membrane having a pore size of 10 to 100 xcexcm;
(9) An apparatus described in (1), wherein the abrasive is silica-based fine particles;
(10) An apparatus described in (2), wherein the abrasive is silica-based fine particles;
(11) An apparatus described in (3), wherein the abrasive is silica-based fine particles;
(12) An apparatus described in (4), wherein the abrasive is silica-based fine particles;
(13) An apparatus described in (5), wherein the abrasive is silica-based fine particles;
(14) An apparatus described in (6), wherein the abrasive is silica-based fine particles;
(15) An apparatus described in (7), wherein the abrasive is silica-based fine particles;
(16) An apparatus described in (8), wherein the abrasive is silica-based fine particles;
(17) An apparatus described in (1), wherein the abrasive is ceria-based fine particles;
(18) An apparatus described in (2), wherein the abrasive is ceria-based fine particles;
(19) An apparatus described in (3), wherein the abrasive is ceria-based fine particles;
(20) An apparatus described in (4), wherein the abrasive is ceria-based fine particles;
(21) An apparatus described in (5), wherein the abrasive is ceria-based fine particles;
(22) An apparatus described in (6), wherein the abrasive is ceria-based fine particles;
(23) An apparatus described in (7), wherein the abrasive is ceria-based fine particles;
(24) An apparatus described in (8), wherein the abrasive is ceria-based fine particles;
(25) A process for recovering abrasive from a waste fluid of a CMP process, the process comprising filtering an entire amount of the waste fluid of a CMP process with a pre-filter to remove coarse particles by catching with the pre-filter, concentrating the filtered waste fluid discharged from the pre-filter by circulating through a microfilter comprising a filtration membrane, washing an obtained concentrated slurry with water to remove impurities soluble in water and fine dusts formed by polishing, concentrating the washed slurry with the microfilter and filtering the concentrated slurry with a post-filter; and
(26) A process described in (25), wherein at least one of pH and a concentration of the slurry washed and concentrated with the microfilter or the slurry obtained by filtering the washed and concentrated slurry with the post-filter is adjusted by adding an agent.